The present disclosure is directed generally to micromachined devices, and in a particular embodiment, to a micromachined rotary actuator capable of being used to precisely position a transducer head within a disk drive.
Various micro-actuation techniques such as electrostatic, thermal, piezoelectric, or magnetic have been demonstrated. Some of the early electrothermal actuator designs are based on the bimorph effect, which relies on the difference of thermal expansion coefficients between two adjacent layers on the device. By heating these layers, a bending moment is created. However such actuators typically produce deflection in the direction normal to the substrate.
U.S. Patent Publication No. 2007/0103029 entitled Self-Assembling MEMS Devices Having Thermal Actuation is directed to a method for designing MEMS micro-movers, particularly suited for, but not limited to, CMOS fabrication techniques, that are capable of large lateral displacement for tuning capacitors, fabricating capacitors, self-assembly of small gaps in CMOS processes, fabricating latching structures, and other applications where lateral micro-positioning on the order of up to 10 μm, or greater, is desired. In self-assembly, motion is induced in specific beams by designing a lateral effective residual stress gradient within the beams. The lateral residual stress gradient arises from purposefully offsetting certain layers of one material versus another material. For example, lower metal layers may be side by side with dielectric layers, both of which are positioned beneath a top metal layer of a CMOS-MEMS beam. In electro-thermal actuation, motion is induced in specific beams by designing a lateral gradient of temperature coefficient of expansion (TCE) within the beams. The lateral TCE gradient is achieved in the same manner as with self-assembly, by purposefully offsetting the lower metal layers with layers of dielectric with respect to the top metal layer of a CMOS-MEMS beam. A heater resistor, usually made from a CMOS polysilicon layer, is embedded into the beam or into an adjacent assembly to heat the beam. When heated, the TCE gradient will cause a stress gradient in the beam, resulting in the electro-thermal actuation.
Turning now to a specific application, the servo system of a disk drive has two primary operations, namely track seek and track follow. Track seek is the operation of moving the head (containing the read transducer and the write transducer) from one data track to another, during which the voice coil motor (VCM) actuator may rotate through its full stroke of 20 to 30 degrees, if one track is at the inner diameter and the other track is at the outer diameter of the disk. After the completion of a track seek, the track follow operation maintains the read or write transducer close to the center of the data track. Challenges to keeping the transducer at the data track center include repeatable and non-repeatable runout of the data track, shock and vibration disturbances, windage disturbances (aerodynamic drag forces arising from laminar and turbulent air flow), and noise in the feedback measurements and electronics.
Head skew is the phenomenon where the longitudinal axis of a read/write head on a disk drive and the tangent of the data track, which the head is reading or writing, are not parallel. That is, the angle between the data track and the head axis is not zero. Head skew degrades the performance of recording in disk drives and is particularly troublesome for disk drives employing perpendicular recording technology, where long narrow poles are desired but cannot be used because they write tracks that are too wide when skewed. Due to curvature of the track the magnitude of the skew is generally less than one-half of the full stroke of the VCM, but can be on the order of 10 degrees.
FIG. 1 illustrates how head skew can be undesirable, particularly in perpendicular recording where head skew results in a wider track width than if the skew were always zero. Shown in FIG. 1A is the position of the head as it would occur at the inner diameter (ID) of the track. Shown in FIG. 1B is the position of the head as it would occur at the middle diameter (MD) of the track and in FIG. 1C as it would occur in the outer diameter (OD) of the track. The need exists for a method and apparatus for eliminating or reducing head skew in disk drives.